Some of the environment variables that are accepted by the
Python interpreter can also be used to affect PL/Python behavior.
They would need to be set in the environment of the main
PostgreSQL server process, for example in a start script. The
available environment variables depend on the version of Python;
see the Python documentation for details. At the time of this
writing, the following environment variables have an affect on
PL/Python, assuming an adequate Python version:

PYTHONHOME

PYTHONPATH

PYTHONY2K

PYTHONOPTIMIZE

PYTHONDEBUG

PYTHONVERBOSE

PYTHONCASEOK

PYTHONDONTWRITEBYTECODE

PYTHONIOENCODING

PYTHONUSERBASE

(It appears to be a Python implementation detail beyond the
control of PL/Python that some of the environment variables
listed on the python man page are only
effective in a command-line interpreter and not an embedded
Python interpreter.)